Phosphor materials have been used in the past, in a variety of forms, to help in alleviating the exposure of patients to X-rays during X-radiation diagnoses.
Storage phosphors, or photostimulable phosphors, have the ability to store some of the X-ray energy and emit longer wavelength radiation when exposed to (excited by) visible light, infrared rays, heat, etc. The secondary excitation needed to cause the emission of longer wavelength radiation in storage phosphors is commonly referred to as stimulation radiation. Storage phosphors are typically used in imaging plates to acquire images in a digital format and to produce images which may be electronically displayed on a CRT, or printed to a suitable hardcopy medium such as photographic film. Typically, in such imaging plates, the storage phosphors are incorporated into a binder which is applied to a suitable support, such as polyethylene terephthalate (PET), and a protective overcoat material is applied over the phosphor containing binder layer.
When ionizing radiations such as X-rays or Gamma rays are used for imaging human bodies it is desirable that the photostimulable phosphors employed in imaging plates emit light at as high a luminescence intensity as possible due to exposure to stimulating radiation after exposure to ionizing radiation so that the dose of ionizing radiations to which the body must be exposed can be reduced. Of the many different and photostimulable phosphor compositions known, many fail to satisfy the practical demands of medical imaging applications by failing to generate sufficient or photostimulated emission intensity for the desired radiation dose. It is therefore desirable to identify storage phosphors which are capable of producing high photostimulated emission intensities.
It would also be desirable to identify useful phosphor hosts having relatively high densities, since, theoretically, for phosphors which have higher densities, thinner phosphor layers could be utilized, resulting in increased resolution. One very popular storage phosphor, BaFBr, has a relatively low density of about 4.96 g/cc.
Phosphors typically consist of a host compound, which is often combined with a small amount of an activator element to change the hue and/or improve the efficiency of fluorescence.
U.S. Pat. Nos. 4,959,174; 4,829,188; and 4,857,741 disclose phosphors in which gadolinium may be combined with at least one divalent metal selected from the group consisting of Be, Mg, Ca, Sr, Ba, Zn and Cd. However, these phosphors also include either or both of Ta and Nb.
Two articles, "On the Eu.sup.3+ Fluorescence of Mixed Metal Oxides," by G. Blasse, J. Chem. Phys., Vol. 47, No. 7, pp. 2356-2360 (October 1966); and "On the Eu.sup.3+ Fluorescence in Mixed metal Oxides," by G. Blasse and A. Bril, Philips Res. Depts. 21, pp. 368-378 (1966), deal with the fluorescence of several gadolinium-containing phosphors, including BaGd.sub.2 O.sub.4 activated by europium. However, there is no mention of photostimulated luminescence or whether other activating elements result in photostimulable properties for this host compound, nor is there any mention that these phosphors are suitable as storage phosphors.
There continues to be a need for materials which are suitable as storage phosphors, particularly those having higher densities and higher photostimulated emission intensity.